Hydraulic fluid containing basic zinc carboxylates

ABSTRACT

HYDRAULIC FLUIDS CONTAINING NOVEL COMPLEXES OF ZINC DIHYDROCARBYL DITHIOPHOSPHATES AND BASIC ZINC CARBOXYLATES ARE NON-CORROSIVE TO SILVER.

United States Patent Oflice 3,726,798 Patented Apr. 10, 1973 3,726,798 HYDRAULIC FLUID CONTAINING BASIC ZINC CARBOXYLATES Howard Bernard Silver, Hinchley Wood, England, as-

signor to The British Petroleum Company Limited, London, England No Drawing. Filed Apr. 14, 1971, Ser. No. 134,077 Claims priority, application Great Britain, Apr. 28, 1970, 20,320/70 Int. Cl. C09k 3/00 US. Cl. 252-75 8 Claims ABSTRACT OF THE DISCLOSURE Hydraulic fluids containing novel complexes of zinc dihydrocarbyl dithiophosphates and basic zinc carboxylates are non-corrosive to silver.

This invention relates to an improved hydraulic fluid.

Hydraulic fluids are fluids intended for use in hydraulic fluid power systems.

A useful source of information on this subject is the book Introduction to Hydraulic Fluids by Roger E. Hatton, published by Reinhold Publishing Corporation, New York, in 1962. Hatton defines a hydraulic fluid power system as a means of power transmission in which a relatively incompressible fluid is used as a power transmitting medium and states that the primary purpose of a hydraulic system is the transfer of energy from one location to another and the conversion of this energy to useful work. He divides the functions of a hydraulic system into the aspects of (a) generation, (b) utilisation, transmission and (d) control of fluid power. Hydraulic power is usually generated by special pumps.

Amongst the many desirable physical and chemical properties of hydraulic fluids are some which unfortunately are conflicting in their requirements. Good load carrying ability is desirable and in order to achieve this it is common practice to add load carrying additives, frequently reactive sulphur compounds such as sulphides, phosphosulphurised hydrocarbons and, in particular, zinc dihydrocarbyl dithiophosphates. The latter are very useful additives because in addition to their load carrying properties they also act as antioxidants.

It is necessary that hydraulic fluids should be noncorrosive towards metallic surfaces with which they come into contact.

Hydraulic pumps may contain slipper pads plated with silver to reduce wear and unfortunately these are attacked by reactive sulphur containing compounds.

Thus it is possible to obtain hydraulic fluids which have satisfactory load-carrying properties but which are corrosive to silver.

It is also possible to obtain hydraulic fluids which are non-corrosive to silver but which have unsatisfactory load carrying properties.

The two conflicting requirements of adequate loadcarrying performance and non-corrosivity to silver pose a difficult problem.

British patent specification 959115 discloses a new class of zinc dithiophosphate additives which are stated to be non-corrosive to silver. The class comprises two types of zinc dithiophosphate salts, viz, 1) normal size bis- (aryl)-dithiophosphates and (2) normal zinc bis-(alkyl) dithiophosphates prepared from so-called neoor hindered-type alcohols, i.e. alcohols in which the carbon atom adjacent to the carbon atom attached to the hydroxyl group is completely substituted with alkyl groups.

I have now discovered novel complexes, the nature of which 'I am not not certain, of zinc dihydrocarbyl dithiophosphates and basic zinc carboxylates which, when added to hydraulic fluids, are substantially non-corrosive to silver.

Thus according to the present invention there is provided a complex of (A) a zinc dihydrocarbyl dithiophosphate of formula (R 0 (R 0) PSSZnSPS (0R (0R wherein R -R are alkyl groups containing 2 to 18 carbon atoms, aryl groups containing 6-12 carbon atoms or alkyl groups containing 7-15 carbon atoms, with (B) a basic zinc carboxylate of formula (R COO) Zn O wherein R is an alkyl, cycloalkyl, aryl, aralkyl or alkaryl group preferably having not more than 24 carbon atoms, the compound having a tetrahedral structure with the 4 zinc atoms at the corners, the oxygen atom in the centre and the 6 carboxylate groups arranged along the sides. Most suitably R is an alkyl group having up to 18 carbon atoms.

Suitable zinc dihydrocarbyl dithiophosphates include zinc di-n-butyl dithiophosphate, zinc di-n-pentyl dithiophosphate, zinc di-isohexyldithiophosphate, zinc di-4- methylpent-Z-yl dithiophosphate, zinc di-p-octylphenyl di thiophosphate and zinc di-isohexylphenyldithiophosphate.

Suitable basic zinc carboxylates are described in British patent specification 1,142,195 and include basic zinc octanoate, basic zinc 2-ethylhexanoate, basic zinc versatate and basic zinc oleate.

The complex may simply be prepared by mixing component (A) with component (B). If the complex is to be used as an additive then the components may be mixed before or after being added to the base stock.

The simple method of preparation contrasts with the method employed to prepare the zinc carboxylate coordinated-zinc dithiophosphate salts of British patent specification 959,115. British patent specification 959,115 discloses a method in which the zinc carboxylate and zinc dithiophosphate are heated in the presence of Water and a hydrocarbon solvent and water is distilled off to obtain a solution of the complex salt.

It is a further advantage of my complex salt that water is not used during its preparation. Water tends to hydrolyse irreversibly complex zinc salts.

The components are suitably mixed in a ratio in the range 1 to 5, preferably 1 to 1.

Zinc dihydrocarbyl dithiophosphates with various hydrocarbyl radicals are readily available from additive ma ufacturers under a variety of trade names.

Basic carboxylates may be prepared by reacting one mole of proportion of zinc oxide w th 1.5 moles of proportions of an acid of formula R COOH, wherein R is as previously defined. Details of the preparation of these compounds are given in British patent specification 1,142,195.

As stated above, the complexes are very suitable for use as load-carrying additives to hydraulic fluids which are substantially non-corrosive to silver.

Thus according to a further feature of the present invention there is provided a hydraulic fluid comprising a major proportion of a base oil and a minor proportion of a complex as hereinbefore described.

Suitably the fluid contains 0.1 to 5%, preferably 0.5 to 2% by weight of the complex.

The base stock may be a mineral oil or a synthetic material.

Suitable mineral oils are parafiinic oils of viscosity in the range 2.0 to 20 cst. at 98.8 C.

The preferred mineral oil is characterized by a viscosity of 5 cst. at 98.8 C.

Suitable synthetic bases include esters of phosphorus containing acids, e.g., triaryl phosphates, silicate esters, halogenated hydrocarbons, e.g. chlorinated diphenyl, esters of organic acids with alcohols or phenols, and polyalkylene glycols.

If the hydraulic fluid is based on a mineral oil, a pour point depressant may be necessary. Suitable pour point depressants include n-alkylated aromatic compounds. They are conveniently used in amount 0.1 to 2% by wt.

It is also very desirable that the hydraulic fluid should contain a small amount e.g., 0.0l0.5% by weight of a rust inhibitor. Suitable rust inhibitors include alkenyl succinic acids, alkyl mercapto acidic acids and alkaryloxy acetic acids.

Other additives which may be used to advantage in specific uses of the hydraulic oil include viscosity index improvers, e.g., polyisobutenes, polyalkylmethacrylates and polyalkyl-styrenes, and antifoam agents, e.g., silicone fluids.

The invention is illustrated by the following examples.

RESULTS OF EMD SILVER CORROSION TEST ON BLENDS OF ZINC DIARYL DITHIOPHOSPHATE (OLOA 260) WITH AND WITHOUT ZINC OARBOXYLATES Example 1 2 3 0102. 260, percent 1.0 1.0 1.0. Plexol 150, percent 0.4 0.4 0.4.

Zinc octanoate, percent" 1.0 Basic zinc octanoate,

percent 1.0.

Base oil, percent 98.6 97.6 97.6.

Appearance of silver plate Grey-black Multi- Dull grey/ after 72 hours. film. colour brown tarnish. tarnish. Weight change, mg -l12, 114. -2.4, 2.8 -0.1, 0.1. Four ball anti-Wear test. 0.29 0.58 36.

Wear scar diameter after 1 hr. With kg. load, mm.

basic zinc carboxylate of formula (R COO) Zn O wherein R is an alkyl, cycloalkyl, aryl, aralkyl or alkaryl group preferably having not more than 24 carbon atoms, the compound having a tetrahedral structure with the 4 zinc atoms at the corners, the oxygen atom in the centre and the 6 carboxylate groups arranged along the sides, the molar ratio of component (A) to component (B) being in the range of 1:1 to 1:5.

2. A hydraulic fluid according to claim 1 which contains 0.5 to 2% by weight of the complex.

3. A hydraulic fluid according to claim 1 in which the mineral oil is a paraffinic oil of viscosity in the range 2 to 20 centistokes at 98.8 C.

4. A hydraulic fluid according to claim 1 in which the mineral oil has a viscosity of 5 centistokes at 98.8 C.

5. The hydraulic fluid as claimed in claim 1 in which R is an alkyl group having up to 18 carbon atoms.

6. The hydraulic fluid as claimed in claim 1 wherein said zinc dihydrocarbyl dithiophos'phate of said complex is zinc di-n-butyl dithiophosphate, zinc di-n-pentyl dithiophosphate, zinc di-isohexyl dithiophosphate, zinc di-4- methylpent-Z-yl dithiophosphate, zinc di-p-octylphenyl dithiophosphate or zinc di-isohexylphenyl dithiophosphate.

7. The hydraulic fluid as claimed in claim 1 wherein said basic zinc carboxylate of said complex is basic zinc octanoate, basic zinc Z-ethylhexanoate, basic zinc versatate or basic zinc oleate.

8. The hydraulic fluid as claimed in claim 1 wherein the molar ratio of (A) to (B) is 1:1 in said complex.

EFFECT OF ZINC CARBOXYLAIES ON ZINC DIALKYL DITHIOPHOSPHATES Example 4 5 6 7 8 9 Plex0l150 0. 0.4 0.4. .4 4 Anglamol75 1.0 1.0.--. 9.0 1 0 Lubrizol1360 1.0 1.0 Normal zinc mrmm 1.0 Basic z inc octoate 1. 0 2. 0.- 1. Base 011 98 6 97 6 97.6 96.6 97.6

EMD silver corrosion test. Appearance Black with Dark brown Black fia Blue re Sli ht tarnish Li ht t l h of silver plate and wt. loss (mg) after green/blue tarnish with deposits tanii sh zvith alinost (5.2; 2???? s 72 hours. tarnish 4 ac (39; 52). flaky solid unchanged 229). gaposits (33; (22; 14). (1.8; 1.7). Four ball wear test 15 kg./1 h./ambient 0. 28- 0.26 0.26 0,31.

(wear scar diameter) mm.

NoTE.-Anglamo1 75 is a rust inhibiting, anti-wear, antioxidant and anti-corrosive package sold by the Lubrizol Corporation. It contains zinc dialkyl dithiophosphate. Lubrizol (R 0) (R 0) PSSZnSPS (0R (0R wherein R -R are alkyl groups containing 2 to 18 carbon atoms, aryl groups containing 6-12 carbon atoms or alkaryl groups containing 7-15 carbon atoms, with (B) a 1360 is a similar composition sold by the Lnbrlzol Corporation. The base oil is as previously described for Examples 1-8.

References Cited UNITED STATES PATENTS 3,328,335 6/1967 Jolie 252-32.75 3,523,081 8/1970 Braid 252- X LEON D. ROSDOL, Primary Examiner H. A. PITLICK, Assistant Examiner U.S. Cl. X.R. 

